400 A. C. LANE — PORPHYRITIC APPEARANCE OF ROCKS 



while it was still viscous, after the marginal zones had largely consoli- 

 dated, but perhaps before the center had cr3^stallized nearly as much. 



Just to fix our ideas, and by way of illustration, if we suppose the 

 countr}^ rock to have had a temperature of 100 degrees centigrade, and 

 that the temperature of formation of the feldspar is 700 degrees centi- 

 grade, then the initial temperature of the magma was 900 degrees. 



In Pirrson's article already referred to, in which I am in full accord, 

 he remarks that the coarseness of grain- and the developement of pheno- 

 crysts must depend on the crystallization interval — in other words, the 

 slowness of cooling. I also agree with him that the loss of included 

 water vapor may be important, but, as he implies, that will be a function 

 of a loss of heat when pressure ceases to change and the eruptive or in- 

 trusive act has ceased. Whether it remains directly proportional to it 

 remains to be seen, but all conclusions as to the belt of uniform grain 

 will be unaffected. The same comment applies to what Pirrson says of 

 viscosit}^, so far as that is directly or indirectly a function in temperature. 



Another case where the same principles apply, although the result is 

 not by any means as striking, is such a diabase dike as that of Light 

 House point, Marquette. In a section which I measured where it is 

 about 54 feet across the augite increased from the margin until about 5 

 feet in (1.622 millimeters) ; the average diameter of the augite was .67 

 millimeters, while at the center it was only a little over 1, wdiereas, 

 where we take the magnetite 5 feet in, that is somewhat less than 2 a 

 millimeter in diameter, where at the center the average dimension is .93 

 millimeters. The result is that the magnetite is relatively much more 

 conspicuous at the center. The inference, confirmed by other observa- 

 tions, is that the magnetite was formed earlier than the augite.* 



Border Phenocrysts or Oriocrystals 



We now come to consider the second class of crystals to which I wish 

 to call special attention, those which are extra large near the margin. 

 In the only case which I have found of which I feel sure the crystals 

 are, however, not what would ordinaril}^ be called phenocrysts, but I 

 take this up to show the possibility, and will then sliow how a slight 

 modification of the initial condition might produce what would })ass for 

 phenocrysts. 



Medford Dike, Massachusetts 



This dike is very similar in general character to the Palisades trap, 

 which has been studied by Queneau, and the Light House Point dike, 



*See following paper. 



